Aflatoxins upregulate CYP3A4 mRNA expression in a process that involves the PXR transcription factor.

Pregnane X receptor (PXR) is a member of the nuclear hormone receptor (NHR) superfamily, which regulates xenobiotic and endobiotic metabolism in the liver. This transcription factor is activated by structurally diverse ligands, including drugs and environmental pollutants. PXR regulates the expression of numerous genes that function in biotransformation and the disposition of xenobiotics upon binding to an AG(G/T)TCA DNA motif in target promoter regions. We performed a screen of mycotoxins that pose a known environmental threat to human and animal health for the ability to activate PXR function in a human hepatocyte cell line, HepG2. We found that aflatoxins B1, M1, and G1 activated PXR. This activation was associated with upregulation of CYP3A4 expression and increased occupancy of PXR protein on the CYP3A4 promoter. Using a microarray approach, we also found that aflatoxin B1 upregulated the expression of multiple genes involved in xenobiotic metabolism, including genes known to be regulated in a PXR-dependent fashion. We also observed an effect of aflatoxin B1 on the expression in other functional groups of genes, including the downregulation of genes involved in cholesterologenesis. The results of this study indicate that aflatoxin B1 is able to activate PXR, a known regulator of liver xenobiotic metabolism, in human hepatocytes, and it can upregulate the expression of PXR-dependent genes responsible for aflatoxin B1 biotransformation, including CYP3A4.